1. Field of the Invention
The present invention relates to a drive circuit and a light emitting device.
2. Description of the Related Art
A light, emitting device that drives a light emitting element such as a laser diode (hereinafter, abbreviated as LD) and a light emitting diode (LED) includes a drive circuit that supplies a driving current to the light emitting element. As a system for generating the driving current, there are known a continuous control system and a switching control system. In the switching control system, control can be performed to cause a current to flow as much as necessary with a pulse modulation signal, so that a circuit loss thereof is smaller than that of the continuous control system, and the switching control system is advantageous in view of power conversion efficiency, a size, and the like. As the switching control system, there is known a synchronous rectification system. The synchronous rectification system is a system for supplying a current to a load such as a light emitting element utilizing a plurality of switching elements the ON/OFF of which is alternately switched with the pulse modulation signal. The synchronous rectification system can achieve power conversion efficiency higher than that of a diode rectification system.
As an example of the diode rectification system, what is disclosed is a drive circuit including a switch circuit arranged in series with an electronic device and a control circuit that performs switching-control on the switch circuit with a pulse modulation signal having a frequency higher than a response frequency of a control target in accordance with a control signal for input (Japanese Unexamined Patent Application Publication No. 05-204470).
The drive circuit of the synchronous rectification system utilizes a capacitor for smoothing a voltage applied to the load. The capacitor accumulates electric charges so that a stable voltage is applied to the load. However, when sufficient electric charges are not accumulated in the capacitor, the electric charge supplied from a power supply is first used for accumulating the electric charges in the capacitor, so that a delay (error) is caused between a control timing at which an output of the driving current to the load is instructed and an output timing at which the driving current is actually output to the load. When sufficient electric charges are not accumulated in the capacitor at the time when output of the drive circuit is started, for example, the switching element and the like may be broken due to generation of a rush current into the capacitor.
The present invention is made in view of such a situation, and prevents a control delay and a rush current from being generated in a drive circuit of a synchronous rectification system.
The accompanying drawings are intended to depict exemplary embodiments of the present invention and should not be interpreted to limit the scope thereof. Identical or similar reference numerals designate identical or similar components throughout the various drawings.